1. Field of the Invention
The present invention relates to an original document reading device employed by a copy machine or the like, particularly to an original document reading device which is capable of automatically detecting the size of an original document.
2. Description of Related Art
Generally, an original document reading device constructed in copy machines or the like can detect the size of original documents automatically. According to detected document size, the size of copy sheet to be supplied is selected with a copy machine.
According to a general method of detecting document size, an original document reading device has a plurality of reflective photo sensors beneath an original document table, and it detects document size according to the light intensity sensed by each photo sensor.
For example, Japanese Laid-Open Patent Application No. 4-66968 discloses a copy machine which has a plurality of photo sensors each consisting of a light emitter and a light sensor which are positioned beneath an original document table. Electric signal is outputted according to the light intensity sensed by the light sensor of each photo sensor, and it is inputted to an input port of a main unit control CPU for controlling the operation of the copy machine. The main unit control CPU detects state of electric signal inputted from each photo sensor, detects the size of an original document, and controls the copy machine according to detected original document size.
However, this method of detecting original document size from the light intensity sensed by each light sensor has the following problem. The light intensity sensed by each light sensor is easily influenced by the light intensity from the light emitter which changes as time passes or by the density of an original document disposed on the document table. Therefore, accuracy of detecting original document size is degraded in a long period.
Also, as set forth above, original document size is generally detected by a control program for main unit control CPU. Therefore, if the number of different document sizes is changed, not only the number of photo sensors but also control program must be adjusted. Once a copy machine is completed and is put on a market; however, original document size which can be detected by the copy machine cannot be modified any longer. For this reason, use of a copy machine is limited.